Historically, organic pigments have been made by coupling in a dilute aqueous slurry. The slurry is then filtered in a filter press to concentrate the pigment and to form a press cake. The press cake is either dried to obtain a dry pigment, or "flushed" to transfer the pigment particles from the aqueous phase to a non-polar oil or resin phase commonly referred to as a "flushed color". In the resulting flushed color state, the pigment particles are kept in a paste-like state for ultimate use in inks or coatings.
Flushing has been carried out for many decades by mixing pigment press cake with suitable types and quantities of "vehicles" which may for example comprise an oil, or a solution of one or more resins and other additives in a solvent. Typically, in the prior art, the transfer is effected by vigorously kneading the press cake and vehicle until the flushing is complete. These are generally batch type operations employing sigma-blade mixers similar to dough mixers or Banbury mixers.
Other prior art techniques also exist. For example, in U.S. Pat. No. 4,309,223, the pigment slurry if forced through a conduit with vehicle injection. U.S. Pat. No. 3,980,488 teaches partially milling dry pigment prior to flushing. U.S. Pat. No. 2,378,786 teaches mixing a pigment slurry with oil, passing the mixture through a colloid mill, which could be considered equivalent to a homogenizer, followed by passing through a heat exchanger and de-watering on a heated belt. U.S. Pat. No. 4,747,882 teaches mixing a pigment slurry with an oil to transfer the pigment from the aqueous phase to the oil.
These processes are very energy and labor intensive. Moreover, the resulting product or dispersion is often an extremely viscous paste which must be laboriously removed by manually shovelling or scraping. Flushed colors may have up to 30-35% by weight pigment, 64.9-69.9% by weight vehicle, and the remainder water. Most of the water in normal flushed color is removed by heat and/or vacuum.
Those skilled in the art will readily recognize that transparency is a highly desirable property of flushed colors During the vigorous kneading step of conventional flushing processes, considerable heat is generated. Heat does not adversely affect the transparent properties of red and blue pigments. However, yellow pigments, particularly AAA yellows, are extremely heat sensitive and can turn from transparent to opaque as a result of being subjected to excessive heat. In order to prevent or at least minimize this problem, the prior art processes employ complex and expensive cooling techniques. Such techniques are not required in the process of the present invention because little, if any, heat is generated.
It will be seen, therefore, that the prior art flushed colors are made by processes which are labor and energy intensive, and in the cases where yellow pigment is being produced, such processes are further complicated by the need for complex and expensive cooling techniques. Once made, the conventional flushed colors are difficult to process further because of their extremely high viscosities.